I. Field of the Invention
The invention relates generally to semiconductor integrated circuits and processes for their manufacture, and more particularly to an intermediate integrated circuit, having a dual layer positive photoresist, and a dual layer positive photoresist process for the manufacture of integrated circuits.
II. Descrpition of the Prior Art
In the manufacture of integrated circuits, it is well known to use a photo lithographic technique to form an image or pattern in a photoresist emulsion on the semiconductor substrate. The pattern then serves as a mask for subsequent etching of the underlying substrate, or for the introduction of dopants into the underlying substrate. The substrate is covered with a layer of photographic emulsion called photoresist and light or other radiation is applied to selected portions of the emulsion. This causes a selective polymerization of the resist. The unpolymerized resist is then developed or dissolved and removed. This leaves a pattern on the surface of the substrate of openings or windows in the photoresist layer. There are two types of photoresist, negative and positive. A negative resist or light hardening resist is one in which light from the exposure process hardens or polymerizes the photoresist. A postive photoresist, or light softened resist, is one in which the light from the exposure process softens or depolymerizes the photoresist. This process, and its place in the overall manufacture of integrated circuits, is known and those who wish more information are referred to the textbooks and articles in the field, for example, Raymond M. Warner, Integrated Circuits (New York: McGraw-Hill, 1965), pp. 150 and 151 in particular. And Peter E. Gise, Semiconductor and Integrated Circuit Fabrication Techniques (Rustern, Virginia: Prentice-Hall, 1970), pp. 104-109. The Warner text describes only negative photoresist, while Gise describes negative and positive. Both books describe also the step of photoresist and photo lithography in the overall manufacture of integrated circuits. Multiple masking and several photo lithographic steps are used in the manufacture of modern integrated circuits.
The present invention relates to the use of positive photoresist. In certain process steps, it is desirable to have a double layer of photoresist over portions of the substrate, and a second layer of photoresist over part of the first layer, but not all of the first layer of photoresist. An example is in the manufacture of programmable roms, it is necessary to provide additional protection during an ion field implant for nitride lines on the substrate. For example, it is desirable to implant with energy of 90 KEV, which is possible with a dual layer photoresist, while with a single layer resist only approximately 35 KEV may be used. The difficulty in using positive photoresist is that the portions of the first layer, which remain and are covered by the second positive photoresist layer and which are exposed to light in the second photo lithographic technique, are subject to being removed by a softened and depolymerized during the exposure of the second photoresist. Further, the portions of the first photoresist layer remaining during the second photoresist may be subject to being dissolved, wrinkled due to heat, lose their dimensional and chemical integrity.
An object of the present invention is to provide a dual layer positive photoresist process in which two layers of positive photoresist may be used one at least partially over the other. Also an object of the invention is intermediate semiconductive device that includes at least two layers of positive photoresist on the substrate with other parts having only the first layer of positive photoresist on the substrate.
Another object of the invention is to provide a repeatable dual layer photoresist process employing positive photoresist that may be used in an industrial setting with commercial acceptable reproducibility and high yield.
A further object of the invention is to provide a dual layer positive photoresist in which the first resist retains its integrity until after the second photoresist layer has been applied and then all the processing steps that are needed are performed until both photoresists are stripped; and in which the second positive photoresist may be satisfactorily applied and used until it has all been removed.